JS `createGraph` modifies objects returned by `load`

```ts
import { createCache } from "jsr:@deno/cache-dir@0.8.0";
import { createGraph } from "jsr:@deno/graph@0.69.7";

const { load } = createCache();
{
// the object returned by `load` is modified,
await createGraph("https://examples.deno.land/hello-world.ts", { load });
}
{
// which can lead to unexpected behavior elsewhere.
const res = await load("https://examples.deno.land/hello-world.ts");
if (res?.kind === "module") {
const content = res.content;
// this should print `true` but actually prints `false`.
console.log(typeof content === "string" || ArrayBuffer.isView(content));
}
}
```

Start at the createGraph entry point and trace how the supplied load callback's result is handled. Reproduce the example with createCache and load, then verify that calling createGraph leaves the returned module content unchanged and that the final type check prints true.

Written by the indexing model from the issue text.